(a) Field of the Invention
The present invention relates to a method for efficiently retransmitting a packet in a mobile communication system. More specifically, the present invention relates to an automatic repeat request (ARQ) method for providing efficient buffer management and efficient scheduling methods
(b) Description of the Related Art
In mobile wireless technology, specifically in a wireless portable internet system, an automatic repeat request (ARQ) algorithm is proposed in order to minimize an error rate and increase error correction efficiency.
The ARQ algorithm is a method for retransmitting a damaged packet with reference to an acknowledgement (ACK) or a negative acknowledgement (NACK) message for each transmitted packet, or with reference to a timeout when the packet is not received.
FIG. 1 shows a block diagram for representing an ARQ transmitter and an ARQ receiver.
A transmission unit 10 transmits a protocol data unit PDU to a receipt unit. The PDU is generated from a service data unit SDU in an upper layer 11 and stored to a buffer 12, and transmitted according to the ARQ mechanism. The PDU includes packet type data.
An ARQ receiver 23 transmits an ARQ feed back message having acknowledgement ACK or negative acknowledgement NACK information according to PDU transmission success or failure.
The ARQ feed back message is used in order to correct an error by using the ARQ in the wireless portable internet system. An ACK message or a NACK message is used as the ARQ feed back message according to the PDU receipt success or failure.
An ARQ transmitter 13 retransmits the PDU or transmits a discard message to the ARQ receiver 23 according to the ACK or NACK message.
As described, data transmission efficiency according to the receipt unit or channel is analyzed by the transmit unit 10 with reference to the receipt success or failure efficiency in the automatic retransmission. Later, the receipt success or failure efficiency is used for quality of service (QoS) analysis and scheduling operation.
FIG. 2 shows a signal flow chart for describing the conventional ARQ method.
When the transmission unit 10 transmits data by using the packet, the receipt unit 20 transmits the ACK message as the ARQ feed back message for the data. There are cumulative and selective ACK messages as the ACK message. The ACK message will be described in detail later.
The ARQ receiver 23 transmits the proper ACK message according to the data packet receipt. However, the data packet is lost and not received by the ARQ receiver when a communication channel quality is bad.
Due to the data packet loss, the ACK message is lost, and therefore the transmission unit 10 receives no ACK message. When the data packet or ACK message is lost, the ARQ transmitter or the ARQ receiver could not wait unlimitedly or retransmit the data. Instead, the data packet is discarded after a predetermined lifetime.
Conventionally, the ARQ transmitter transmits a discard message in step S1 when the data packet or the ACK message is lost and the transmission unit receives no ACK message for the predetermined lifetime.
The ARQ receiver transmits a discard response message in response to the discard message, and the ARQ transmitter discards the transmitted data packet in step S2 when the ARQ transmitter receives the discard response message.
FIGS. 3 to 5 respectively show ARQ feed back message types in the wireless portable internet system.
FIG. 3 shows a selective ACK message.
As shown in FIG. 3, it is assumed that 12 PDUs are transmitted to the receive unit in a medium access control (MAC) layer of the wireless portable internet system, and errors occur in the PDUs corresponding to the fourth, seventh, eighth, and twelfth sequence numbers. The receiver in the receive unit transmits an ACK MAP as the ARQ feed back message in response to the PDUs. A receipt success is mapped as 1 and a receipt error or failure is mapped as 0 in the ACK MAP. The PDU corresponding to the sequence number received by the ARQ transmitter and mapped as 0 is retransmitted according to the ACK MAP.
FIG. 4 shows a cumulative ACK message.
When the errors occur in the PDUs corresponding to the fourth, seventh, eighth, and twelfth sequence numbers as in FIG. 3, the ARQ receiver records up to the sequence number of the PDUs which were successfully received, and frames the ARQ feedback message in FIG. 4.
Accordingly, the PDU corresponding to the third sequence number is notified to be successfully received, and the ARQ transmitter retransmits the PDUs corresponding to the fourth sequence to the twelfth sequence numbers.
FIG. 5 shows a selective-cumulative mixed ACK message.
While the selective ACK message shown in FIG. 3 is efficient because it retransmits the PDUs having a receipt error, a data processing time and the message of the ACK map are problematically increased. While the cumulative ACK message shown in FIG. 4 has a smaller message size and is a higher speed process, data to be retransmitted are problematically increased.
FIG. 5 shows a method requesting retransmission by framing the sequence number of the successfully received PDUs with an ACK map after the sequence number. The method combines advantages of the selective ACK message and the cumulative ACK message.
As described above, in the prior art, the discard message is transmitted when the predetermined lifetime is passed after the data is transmitted, and the discard operation is completed when the discard response message for the discard message is received. Accordingly, an additional feedback message is required for the discard operation beside the ACK message.
An appropriate scheduling operation is not performed when the ARQ transmitter entering the discard state after the predetermined time is correspondingly treated with the ARQ transmitter entering the discard state by the transmission failure (NACK). At this time, a block loss rate for a session is not accurately calculated because the discard information of the QoS contains fallacy statistics.